The present disclosure relates to an optical scanning device that forms a latent image on a surface to be scanned by exposure scanning, and an image forming apparatus including the same such as a copy machine, a printer, a facsimile, and a multifunctional peripheral thereof.
An optical scanning device used in a copy machine, a printer and the like generally includes a light source, a polygon mirror (a rotating polygon mirror) that deflects and scans beam light from the light source, a driving motor that rotates the polygon mirror, a fθ lens that forms an image of the beam light deflected by the polygon mirror on a surface to be scanned, a folding mirror that returns the beam light having passed through the fθ lens so as to be led to an image carrying member, and a housing to which these elements are fixed.
The housing has a bottom surface portion to which the polygon mirror, the fθ lens, and the folding mirror are fixed, and a side wall portion vertically installed from an edge portion of the bottom surface portion. The beam light output from the light source passes through the polygon mirror and the fθ lens, is returned by the folding mirror, and then is irradiated toward a photosensitive drum (an image carrying member) via an opening formed in the bottom surface portion of the housing.
Since the orientation (a mounting angle) of the folding mirror has an influence on an irradiation position of the beam light to the photosensitive drum and an image position on a sheet (a recording medium), high dimension accuracy is required in support portions of the housing, which support the folding mirror. Therefore, it is general that support surfaces of the support portions are formed as surfaces as small as possible and the folding mirror is supported by three support portions. In such a configuration, typically, three points of vertical two points of one end portion of a reflective surface of the folding mirror in a longitudinal direction and one point of the center of the other end portion thereof are supported by the support portions.
For example, as with an apparatus compatible with an A3 size, when an area for deflecting and scanning light is wide or when a distance from the photosensitive drum to the folding mirror is short, the folding mirror becomes long. Therefore, since a natural frequency of the folding mirror is reduced, there is a case in which the natural frequency of the folding mirror approximately coincides with a driving frequency of the driving motor and thus the folding mirror resonates with the driving motor.
In order to avoid such a problem, it is necessary to decrease or increase the natural frequency of the folding mirror and thereby to allow the natural frequency of the folding mirror to be kept away from the driving frequency of the driving motor. In this regard, there has been proposed an optical scanning device that decreases the natural frequency of the folding mirror by sticking a weight to the folding mirror. However, in the case of sticking a weight to the folding mirror, there is a case in which an assembling process is increased and the folding mirror is bent by the weight. Furthermore, since many other driving motors driven with driving frequencies lower than that of the driving motor for driving the polygon motor exist in an image forming apparatus, even though the natural frequency of the folding mirror is decreased by sticking a weight to the folding mirror, there is a case in which the other driving motors and the folding mirror resonate with each other.
In this regard, there has been proposed an optical scanning device that increases the natural frequency of the folding mirror by supporting each one point of one end portion, a center portion, and the other end portion of the reflective surface the folding mirror in the longitudinal direction. In this optical scanning device, the one end portion and the other end portion of the reflective surface in the longitudinal direction are supported by, for example, an upper side in a transverse direction, and the center portion of the reflective surface in the longitudinal direction is supported by, for example, a lower side in the transverse direction, so that three point support is realized. In addition, in this optical scanning device, since an approximately center of the reflective surface in the longitudinal direction is supported and thus a distance between the support points is reduced to about ½, the natural frequency of the folding mirror is increased about twice.